Catalytic metathesis

In contrast, substituting the ort/to-methyl groups of SIMes with ortho-fluoride atoms profoundly alters the catalytic metathesis performance. In 2006, Grubbs and co-workers reported the synthesis of the fluorinated NHC-Ru catalysts 25 and 26 [41] (Fig. 3.8). Catalytic tests in the RCM of 1 to form 2 showed that the phosphine-free catalyst 26 was slower than the standard catalyst 16, which was consistent with theoretical investigations suggesting the electron-withdrawing fluoride atoms would lead to a decrease in catalyst activity [42]. However, in contrast to the computational... 

Keywords Catalytic metathesis, Enantioselective synthesis, Catalytic kinetic resolution, Catalytic carbomagnesation, Catalytic rearrangement, 2-Substituted chromenes, Catalytic m aero cyclization... 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

Catalytic metathesis has been accomplished via a tandem combination of catalytic alkane dehydrogenation, olefin metathesis and subsequent olefin hydrogenation (Scheme 12.6). Two factors are crucial for this transformation ... 

A breakthrough in catalytic metathesis applications was achieved with the second generation of ruthenium-NHC-alkylidene complexes In 54, 55, and 56 NHCs are combined with coordinatively more labile ligands such as phosphines... 

The mechanism of the catalytic metathesis reaction proceeds via reaction of the olefin substrate with a metal carbene intermediate, which may be generated in situ... 

The only direct evidence for the presence of metal-carbene-olefin intermediates in catalytic metathesis systems comes from a study of the interaction of the tungsten cyclopentylidene complex 27 with cycloalkenes such as cycloheptene 28 in CD2CI2. When these are mixed at —96 °C and the temperature raised to between —53 and —28 °C, no polymerization occurs but the 13C NMR spectrum contains additional resonances which may be assigned to the metal-carbene-olefin complex 29. The line intensities show that the equilibrium 7 moves to the right as the temperature is lowered120. 

Three Nobel Prize lectures have featured the metathesis reaction in detail, focusing on early results and development from a mechanistic point of view,2 the development of catalytic metathesis reactions,3 and the use of metathesis in the preparation of target molecules.4... 

Schrock, R.R. (2006) Multiple metal-carbon bonds for catalytic metathesis reactions (Nobel Lecture). Angew. Chem. Int. Ed., 45, 3748. 

Several mechanisms were proposed to explain the catalytic metathesis reactions, but the mechanism published by Yves Chauvin in 1971 has come to be accepted as correct. We can think of an alkene as two alkylidene groups bonded together. Similarly, the Schrock and Grubbs catalysts are like a metal atom bonded to one alkylidene group. 

We have developed solvothermal synthesis as an important method in research of metastable structures. In the benzene-thermal synthesis of nanocrystalline GaN at 280°C through the metathesis reaction of GaClj and U3N, the ultrahigh pressure rocksalt type GaN metastable phase, which was previously prepared at 37 GPa, was obtained at ambient condition [5]. Diamond crystallites were prepared from catalytic reduction of CCI4 by metallic sodium in an autoclave at 700°C (Fig.l) [6]. In our recent studies, diamond was also prepared via the solvothermal process. In the solvothermal catalytic metathesis reaction of carbides of transition metals and CX4 (X = F, Cl, Br) at 600-700°C, Raman spectrum of the prepared sample shows a sharp peak at 1330 cm" (Fig. 1), indicating existence of diamond. In another process, multiwalled carbon nanotubes were synthesized at 350°C by the solvothermal catalytic reaction of CgCle with metallic potassium (Fig. 2) [7]. 

Figure 1. Raman spectra of the sample prepared by solvothermal process through the catalytic reduction [6] (left) and the catalytic metathesis reaction (right).

Schrock, R. R. Multiple Metal-Carbon Bonds for Catalytic Metathesis Reactions (Nobel Lecture 2005). Adv. Synth. Catal. 135. 2007 349 41-53. 

It has been found that supported molybdenum oxide catalyzes an efficient catalytic metathesis reaction. The metathesis of C3H6 at 473 K on the molybdenum oxide/Si02 catalyst led to the products ethene, 1-butene, and CIS- and rram-2-butene, the ratio of the three butenes being near the equilibrium ratio at the reaction temperature of 473 K. As shown in Fig. 27, with an increase in the Mo content the rate of the metathesis reaction increases, passing through a maximum at approximately 2 atom% Mo. This feature closely resembles the change in the concentration of the tetrahe-drally coordinated dioxo-molybdenum oxide species 138, 153). 

The metathesis of alkynes by metal alkylidyne complexes is now a well-established process. The reaction is useful for the catalytic metathesis of alkynes as well as for the synthesis of new metal alkylidyne complexes. The general electronic and steric factors favoring the metathesis reaction were discussed in recent reviews by Schrock (6,7). 

Kotha, S., Sreenivasaohary, N. Catalytic metathesis reaction in organic synthesis. Indian J. Chem., Sect. B 2001, 40B, 763-780. 

EPR spectroscopy has been applied by Kucherov and coworkers to study the nature of the catalyticaUy active species in [W +(Mo"+) - ionic liquids - olefin] metathesis systems [77]. In this study the key role of isolated paramagnetic complexes of W + and Mo + ions in the catalytic metathesis of 1-hexene could be demonstrated. It... 

As mentioned in earlier sections of this chapter, alkylidene complexes are crucial intermediates in olefin metathesis, and this process occurs by sequential [2+2] reactions of olefins or alkynes with the metal-carbene complex. Catalytic metathesis of olefins and... 

Overview of the Catalytic Metathesis of Carbon-Carbon Multiple Bonds... 

Schrock et al. studied intensively the stoichiometric and catalytic metathesis of disubstituted alkynes with the carbyne W(VI) complex ( BuO)3WsC Bu, Figure 1. They also tested some monosubstituted alkynes and found that they form polyalkynes... 

Encouraged by our results on stoichiometric and catalytic metathesis reactions of carbodiimides and imines with Fischer type carbene tungsten(O) complexes (9) we started 1984 with metathesis like reactions of the Schrock type carbyne tungsten (VI) complex Cl3(dme)WCtBu with heteroallenes (isocyanates, carbodiimides, isothiocyanates) and with heteroalkenes (imines and nitroso compounds). 

Cl3(dme)WCtBu gives not only stoichiometric but also catalytic metathesis reactions with differently substituted carbodiimides or imines (10) ... 

Following the catalytic metathesis of carbodiimides and isothiocyanates with Cl3(dme)WCtBu, we tested the metatheses of alkenes with this complex. Linear 1-alkenes and monocyclic alkenes (scheme 4) give catalytic metathesis with the Schrock type carbyne complex in CH2CI2 at 20°C (12). At 76 °C and in 1,2 dichloroethane as solvent the turnover frequence enhance (Table 1) (13). 

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